Hi Hal,
Sizes can vary, of course. Here is some information which gives you general idea about how to convert Megapixels to Megabytes.
"The size of the digital file corresponding to the image which the camera produces depends on the pixel count. In most consumer digicams each pixel generates 3 bytes of data (so called "8-bit data"). One for red, one for green and one for blue. This means that a 3MP camera, which has 3 million pixels, generates 9 million bytes of data, or 9MB (megabytes). A few cameras can generate extra data for extra quality, and some of these cameras generate files which correspond to 2 bytes of data for each color ("16-bit"), so a 3MP camera which is capable of generating 16-bit data will produce an 18MB image file."
Source: Photonet (see JPEG, TIFF & RAW)
http://www.photo.net/equipment/digital/basics/"A typical 6-megapixel digital SLR camera produces images with pixel dimensions requiring 18 megabytes. If the photographer takes advantage of RAW capture capabilities, resulting in a 16-bit file, that size doubles to 36MB. Resizing the image for a 20”x30” print results in a file of around 150MB in 8-bit, or 300MB in 16-bit."
Source: Microsoft
http://www.microsoft.com/windowsxp/using/digitalphotography/expert/optimizedsystem.mspx
